Noise chopper for preventing false locks in a coherent carrier recovery system

DETAILED DESCRIPTION We claim: 1.
In a multiple phase shift keyed data coherent carrier recovery system including a coherent carrier tracking loop, apparatus comprising a signal generator providing a signal with non-stationary zero-crossing times and means for connecting said signal generator to the loop for altering the phase of signals therein and preventing the loop from locking onto a harmonic or subharmonic of the signal generator.
2.
Apparatus as claimed in claim 1 wherein the signal generator is a noise generator.
3.
Apparatus as claimed in claim 1 wherein the connecting means is a multiplier having an output connected to the input of the recovery system, an input connected to receive the input signal for the system and an input connected to receive the signal from the signal generator.
4.
A multiple phase shift keyed data coherent carrier recovery system comprising a coherent carrier tracking loop and a signal generator for providing a signal with non-stationary zero-crossing times with means connecting said signal generator to said loop for chopping signals in said loop.
5.
A system as claimed in claim 4 wherein the tracking loop includes a Costas demodulator loop.
6.
In a multiple phase shift keyed data coherent carrier recovery system including a coherent carrier tracking loop, a method of preventing the loop from locking into a harmonic or subharmonic of a chopper comprising the steps of aperiodically reversing the phase of signals in the tracking loop.




Description:
BACKGROUND OF THE INVENTION Demodulators or coherent carrier trackers, particularly for phase shift keyed signals, have two basic problems during acquisition; one theoretical, one practical.
When acquiring a modulated signal in a demodulator with a control loop bandwidth that is narrow compared to the modulation rate, the loop may very likely synchronize to a subharmonic of the modulation rate above or below the desired carrier frequency synchronization point.
This can occur in high gain loops whenever the channel or the demodulator is band limited, i